In recent years, electronic apparatus such as portable wireless telephone, portable personal computer and portable video camera have been developed, and various electronic apparatus have been reduced in size to an extent such that they are portable. This requires the use of batteries having a high energy density and a light weight as batteries to be incorporated in these electronic apparatus. A typical battery that satisfies these requirements is a nonaqueous secondary electrolytic battery comprising as a negative electrode material a lithium intercalation compound having lithium metal, lithium alloy or lithium ion occluded in a carbon-based material as a host material (The term “host material” as used herein is meant to indicate a material capable of occluding and releasing lithium ion), as a positive electrode material a compound which undergoes reversible electrochemical reaction with lithium ion such as lithium-cobalt composite oxide and as an electrolytic solution an aprotic organic solvent having a lithium salt such as LiClO4 and LiPF6 dissolved therein.
This nonaqueous secondary electrolytic battery comprises a negative electrode plate having a negative electrode compound containing the foregoing negative electrode material retained on a negative electrode collector as a support, a positive electrode plate having a positive electrode compound containing the foregoing positive electrode material retained on a positive electrode collector as a support and a separating material provided interposed between the negative electrode plate and the positive electrode plate. The separating material often means a separator for retaining the electrolytic solution as well as interposing between the negative electrode plate and the positive electrode plate to prevent the shortcircuiting of the two electrodes. However, a solid electrolyte which is disposed between the negative electrode plate and the positive electrode plate to prevent shortcircuiting as well as cause ionic conduction, too, is called a separating material.
The foregoing positive electrode plate and negative electrode plate, which are each in the form of thin sheet, are laminated on each other or spirally wound normally with a separating material interposed therebetween to form an electricity-generating element. The electricity-generating element thus is received in a battery case made of a metal such as stainless steel, nickel-plated iron or aluminum into which an electrolytic solution is then injected. The battery case is then hermetically sealed with a cover plate to assemble a battery.
The use of such a metallic battery case provides a high airtightness and an excellent mechanical strength but puts great restrictions on the reduction of the weight of the battery and the selection of the shape of the battery.
As an approach for solving the foregoing problem there has been proposed a structure comprising an electricity-generating element received in a battery case made of a resin sheet. This structure is advantageous in that it can reduce the weight of the battery and enhance the degree of selection of the shape of the battery. On the other hand, this structure has the following disadvantages and problems awaiting solution.
(a) A structure comprising as the foregoing resin sheet a resin sheet made of a laminate of a metal foil and a heat-fusible resin layer is advantageous particularly in that it gives a high airtightness. However, when the resin sheet is welded under tension, the metal foil undergoes cracking, deteriorating the sealing properties of the resin sheet. Thus, water enters into the battery, reducing the life of the battery.
(b) The structure having a battery case formed by this kind of a resin sheet is liable to peeling of the welded portion of the ends of the sheet which are superimposed on each other with the rise in the inner pressure of the battery. The metal-resin sheet exhibits a sufficiently enhanced airtightness in the direction perpendicular to the metal foil. However, the exterior and the interior of the battery case are separated not by the metal foil but by the resin layer sandwiched by two sheets of metal foil at the welded portion of the ends of the sheet which are superimposed on each other. Thus, water content and electrolytic components can easily pass though the resin layer.
(c) The production of this kind of a battery is often carried out by a process which comprises putting an electricity-generating element provided with lead terminals into an open battery case made of a resin sheet, injecting an electrolytic solution into the battery case, and then welding the opening portion of the battery case with the lead terminals put between the two sides of the opening portion to seal the battery case. However, since the electrolytic solution hits and bounces off the electricity-generating element or is attached to the lead terminals during injection into the battery case, welding is occasionally carried out with the electrolytic solution attached to the area of the resin sheet to be welded. This also causes the deterioration of the sealing properties.
(d) The spirally-wound electricity-generating element, if used, can be easily loosed when received in a battery case made of a resin sheet unlike in the metallic case. Eventually, the gap between the various electrode plates cannot be kept constant over an extended period of time. Accordingly, this structure tends to show an decrease of discharge capacity after repeated charge-discharge cycles.
(e) At the first step of forming a battery case from a resin sheet, two parallel sides of the resin sheet are welded to each other to form a cylinder. However, the welded portion protrudes from the battery case when the battery is assembled. Thus, this protrusion interferes in the accumulation of a plurality of the batteries. Accordingly, the accumulation of these batteries can easily produce an unnecessary space or takes time to compress the stack of these batteries so that the protrusion doesn't interfere.
(f) In this structure, the electricity-generating element is subject to less pressure than the conventional case comprising a metallic case. Accordingly, in the case where the battery comprising a case made of a resin sheet is used in a vibrational atmosphere, the electricity-generating element can easily move in the battery case, occasionally breaking the lead terminals at the point between the fixed area at the heat-welded portion and the electricity-generating element. Further, since the electricity-generating element is subject to small pressure, there occurs nonuniformity in the distance between electrodes in the electricity-generating element, causing the deterioration of charge-discharge properties.
The present invention has been worked out in the light of the foregoing circumstances. An object of the present invention is to provide a battery having a long life which exhibits a sufficiently enhanced airtightness while attaining the reduction of the weight of the battery case made of a resin sheet. Another object of the present invention is to prevent the welded portion of the two ends of resin sheet from interfering in the accumulation of batteries, enhancing the volume energy density of the entire battery. A still another object of the present invention is to prevent the breakage of lead terminals in the battery case.